1. Field of the Invention
This invention relates to lead-free piezoelectric ceramic films and a method of making them. Specifically, the invention may be particularly beneficial for use in microelectronic devices such as energy harvesting devices and sensor technologies.
2. Description of the Related Technology
Due to their lead content, current lead based piezoelectric materials and devices are highly toxic. Alternative lead-free piezoelectric materials, however, typically have inadequately low piezoelectric coefficients and thus require complex and expensive methods to provide enhanced piezoelectric activity in order to make them useful. Generally, such methods involve one or more of: texturing the material to align the grains, seeding of plate-like seeds within the green bodies or complex sintering schemes.
Among the lead-free piezoelectric materials, sodium potassium niobate [(Na0 5K0 5)NbO3, NKN] offers a high Curie temperature (Tc≅420° C.) and a relatively high piezoelectric coefficient (d33=80-160 μC/N, −d31=32-49 pC/N) (See R. E. Jaeger and L. Egerton, “Hot-Pressing of Potassium-Sodium Niobates,” J. Am. Ceram. Soc. 45, 209 (1962); H. Birol, D. Damjanovic and N. Setter, “Preparation and Characterization of (K0 5Na0 5)NbO3 Ceramics”, J. Eur. Ceram. Soc. 26, 861 (2006)). To further increase the piezoelectric coefficient, scientists have experimented with various methods for processing solid solutions of NKN with a tetragonal phase-inducing component such as LiNbO3 (LN) and LiTaO3 (LT). By cold isostatic pressing (CIP) this mixture before sintering, the d33 coefficient may be increased to about 200 pC/N at the morphotropic phase boundary (MPB) (See Y. Guo, et al., “Phase Transitional Behavior and Piezoelectric Properties of (Na0.5K0.5)NbO3—LiNbO3 Ceramics,” Appl Phys. Lett, 85, 4121 (2004); Y. Guo, et al., “(Na0.5K0.5)NbO3—LiTaO3 Lead-free Piezoelectric Ceramics,” Mater. Lett, 59, 241 (2005)) Mixing NKN, LN, LT and antimony and using a texturing technique, the piezoelectric coefficient may further be increased, wherein d33=416 μC/N and −d31=152 pC/N (See Y. Saito, H., et al., “Lead-free Piezoceramics,” Nature, 432, 84 (2004)). To date, 416 pC/N is the best d33 coefficient of all lead-free piezoelectric ceramics, which is comparable with the d33 coefficient of dominant lead-based piezoelectric materials such as lead zirconate titanate (PZT). However, the methods required to obtain these desirable piezoelectric coefficients are too complex and expensive and consequentially, not suitable for commercialization.
Therefore, there exists a need to develop alternative lead-free piezoelectric materials, more specifically free standing lead-free piezoelectric films having enhanced piezoelectric coefficients.